The Mode of Action of Sodium Hypochlorite in the Decolorization of Azo Dye Orange II in Aqueous Solution

The effect of sodium hypochlorite (NaOCl) on the decolorization of azo dye orange II (4-[(2-hydroxy-1-naphthalenyl) azo]-benzenesulfonic acid, monosodium salt) in aqueous solution was studied as a function of pH. The first-order rate constant of color reduction (k) was evaluated from the curve of color reduction in the initial stage of decolorization. The k values increased with increasing the pH from 5.4 to 9.3 and the NaOCl concentration from 2.82 x 10(-4) M to 1.13 x 10(-3) M. It was found that k depended on the concentration of OCl-, but not on HOCl. On the other hand, k decreased markedly at pHs more than 10, depending on pH. This suggested that the reactivity of the orange II molecule with OCl- decreased probably due to the competitive action of the hydroxide ion under alkaline conditions. It could be concluded that controlling the solution pH and the OCl- concentration was indeed a key factor determining the rate of decolorization of orange II in aqueous solution.